A display apparatus using a liquid crystal display (also called a liquid crystal display panel), an electroluminescence device (classified into an organic electroluminescence device and an inorganic electroluminescence device depending on a fluorescent material used), field emission device (FED), an electrophoresis device, etc. can display an image without providing a space (vacuum basket-like body) where an electron beam is scanned in a two-dimension behind a display screen, like a cathode ray tube (CRT). Accordingly, such a display apparatus has an advantage of thinness, light weight, low power consumption, etc. over the CRT. This display apparatus is also called a flat panel display from its feature.
The display apparatus using the liquid crystal display, the EL device or the FED device has spread to replace the CRT in various fields including OA equipments such as notebook PCs, monitors for PC and so on, mobile terminals, televisions, etc. because of the advantage over the CRT. The replacement of the CRT with the flat panel display is based on technological innovation in improvement of image quality such as spread of viewing angle or spread of an area of display color reproducibility of the liquid crystal display or the EL device. In addition, in recent years, display quality of moving pictures has been improved as multimedia and Internet have widely spread. In addition, there appear new fields including electronic paper or large scaled information display for public interests or advertisement, which can not be realized by the CRT.
A liquid crystal display comprises a liquid crystal cell, a driving circuit that applies a display signal voltage to the liquid crystal cell, a backlight (a back light source), and a signal control system that transmits an input image signal to the driving circuit, which are also collectively called a liquid crystal module.
The liquid crystal cell includes liquid crystal molecules, two substrates that seal and hold the liquid crystal molecules, and electrode layers that apply a voltage to the liquid crystal molecules. A polarizing plate is disposed in the outside of the liquid crystal cell. The polarizing plate comprises a protective layer and a polarizer made of a polyvinyl alcohol film. Specifically, the polarizing plate is obtained by dyeing the polarizer with iodine, expands the dyed polarizer, and stacking the protective layer on both sides of the expanded polarizer. In case of a transmission type liquid crystal display, this polarizing plate is attached to both sides of the liquid crystal cell, and one or more optical compensation sheets may be further provided. In addition, in case of a reflection type liquid crystal display, a reflecting plate, a liquid crystal cell, one or more optical compensation sheets, and a polarizing plate are typically arranged in order.
The liquid crystal cell performs ON/OFF display according to alignment conditions of the liquid crystal molecules. For the liquid crystal cell, there have been proposed display modes such as TN (Twisted Nematic), IPS (In-Plane Switching), OCB (Optically Compensatory Bend), VA (Vertically Aligned), and ECB (Electrically Controller Birefringence) modes, which can be applied to both of the transmission type liquid crystal display and the reflection type liquid crystal display.
An optical compensation film is used to alleviate image coloring or extend a viewing angle in liquid crystal displays. An expansible birefringent polymer film has been used as the optical compensation film. Alternatively, in addition to the optical compensation film comprising the expansible birefringent polymer film, there has been proposed to use an optical compensation film having an optical compensation layer formed of low molecule or high molecule liquid crystals on a transparent support. Since the liquid crystal molecules have various forms of alignment, use of the liquid crystal molecules allows realization of optical properties which can not be obtained by the conventional expansible birefringent polymer film. In addition, there has been proposed a structure that has both of functions of a protective layer and an optical compensation film by adding birefringence to the protective layer of a polarizing plate.
An optical property of an optical compensation film depends on an optical property of a liquid crystal cell, specifically, a display mode. Use of liquid crystal molecules allow manufacture of optical compensation films having various optical properties corresponding to different display modes of the liquid crystal cell. There have been already proposed optical compensation films using liquid crystal molecules corresponding to various display modes.
For example, an optical compensation film for the TN type liquid crystal cell improves a viewing angle characteristic of contrast by prevention of light leakage in an inclined direction in black color display by making an optical compensation of a tilted alignment state for a substrate while restoring a twisted structure of liquid crystal molecules by application of a voltage (see JP-A-6-214116 and JP-A-8-50206). An optical compensation film for parallel alignment makes an optical compensation of liquid crystal molecules aligned in parallel to a substrate and improves a viewing angle characteristic of perpendicular transmittance of a polarizing plate in black color display under application of no voltage (see Japanese Patent No 3342417).
However, even when an optical compensation film made by hybrid-aligning discotic liquid crystal compounds uniformly is used, it is very difficult to fully compensate a liquid crystal cell optically. For example, when the TN type liquid crystal cell is observed from an inclined direction, there occurs a gray scale inversion effect that transmittance in each gray scale is inverted. As one of methods for preventing the gray scale inversion effect, there has been known a method for limiting a range of a tilt angle of liquid crystal molecules in a liquid crystal cell (see “Technical Report of IEICE”, EID 2001-108, p. 47-52).
In addition, under progress of improvement of display quality of liquid crystal displays, as one of problems of viewing angle characteristics, a problem of a difference between a γ characteristic in front viewing and a γ characteristic in oblique viewing, that is, a problem of dependency of a γ characteristic on a viewing angle, has issued at present Here, the γ characteristic refers to dependency of display luminance on a gray scale. Since the difference between the γ characteristic in front viewing and the γ characteristic in oblique viewing means that the gray scale depends on a viewing direction, there may occur a particular problem in case of display of photographs and the like or TV broadcasting. There has been proposed various liquid crystal displays with an improved viewing angle characteristic of the γ characteristic. For example, Patent Document 4 discloses a liquid crystal display of a normally black mode with improved dependency of a γ characteristic on a viewing angle. In addition, in an ECB mode with high transmittance and high response speed, there is a need to improve a viewing angle characteristic by lessening dependency of a γ characteristic on a viewing angle.
On the other hand, although the above-mentioned methods improve the viewing angle characteristic, there occurs a problem of contraction of a polarizing plate and light leakage at a circumference of the polarizing plate under severe use environments, for example, high temperature or high humidity environments.
In order to overcome the problem related to durability of the polarizing plate, JP-A-7-191217 and EP 911656 disclose a technique in which an optical compensation sheet made by applying an optically anisotropic layer made of a discotic (disk-like) compound on a transparent support is directly used as a polarizing plate protection film without increasing thickness of a liquid crystal display.
On the other hand, there has been proposed a technique for overcoming the light leakage problem by properly selecting an adhesive material of the polarizing plate (see JP-A-2004-216359).
In addition, in order to overcome the durability problem, JP-A-2001-264538 discloses a technique in which the product of an photoelastic coefficient of an optical compensation sheet and an elasticity coefficient of an adhesive layer is set to be less than 1.2×10−5, JP-A-2001-272542 discloses a technique in which an elasticity coefficient of an adhesive layer is set to be less than 0.06 MPa, JP-A-2002-122739 discloses a technique in which the product of a linear expansion coefficient of a polarizing plate protection layer and an elasticity coefficient of an adhesive layer is set to be less than 1.0×10−5 (° C.−1·MPa), and Patent Document 11 discloses a technique in which the product of an photoelastic coefficient of a polarizing plate protection layer and an elasticity coefficient of an adhesive layer is set to be less than 8.0×10−12 (m2/N·MPa).